What BMI measures and what it does not
Body mass index, or BMI, is a ratio of weight to height. It is calculated as weight in kilograms divided by height in meters squared. The US Centers for Disease Control and Prevention (CDC) describes it as a screening measure: a quick, cheap number that can point to possible weight-related risk, not a diagnosis of body fatness or health. It cannot tell muscle from fat, it says nothing about where fat is carried, and it does not account for age beyond the adult cut-offs, sex, ethnicity or fitness.
This calculator is for adults aged 20 and older. For children and teens aged 2 to 19, the CDC compares BMI with growth-chart percentiles for the same age and sex, which this page does not do. If the person is under 20, use a pediatric growth chart with a clinician.
The BMI formula
BMI = weight (kg) ÷ height (m)². With US units the same ratio is 703 × weight (lb) ÷ height (in)², but this calculator converts to kilograms and meters at full precision first and only rounds what it displays.
Worked example: an adult who is 175 cm tall and weighs 70 kg has a BMI of 70 ÷ 1.75² = 22.9 (more precisely 22.8571429), which falls in the healthy weight category. At 95 kg and the same height the BMI is 31.0 (obesity, class 1); at 55 kg it is 18.0 (underweight).
CDC adult BMI categories
For adults aged 20 and older, the CDC uses these standard weight-status categories: below 18.5 is underweight, 18.5 to less than 25 is healthy weight, 25 to less than 30 is overweight, and 30 or higher is obesity. Obesity is further split into class 1 (30 to less than 35), class 2 (35 to less than 40) and class 3, severe obesity (40 or higher). Each lower bound belongs to the higher category, so a BMI of exactly 25 is overweight and exactly 18.5 is healthy weight.
Why the category uses the unrounded value
BMI is usually quoted to one decimal place, but a category boundary is a sharp line. A person at 175 cm and 76.5625 kg has a BMI of exactly 25.0000, which is overweight; at 76.5 kg the BMI is 24.9796, which rounds to 25.0 yet is still healthy weight. This calculator always decides the category from the unrounded value and tells you when rounding would otherwise hide the difference.
Limits you should know about
- Muscle and fat. A muscular person can have a high BMI with little excess fat. Someone who has lost muscle can have a normal BMI with high body fat.
- Fat distribution. Fat carried around the waist carries different risk from fat elsewhere, which BMI cannot see. A waist measurement adds information; see the waist-to-height calculator for the ratio arithmetic.
- Population differences. The cut-offs were set from studies of mostly general populations. Some health bodies use different thresholds for some groups, so ask a clinician what applies to you.
- Age. For older adults, a slightly higher BMI is not always unhealthy and unintended weight loss can matter more than the number.
- Pregnancy. The adult categories do not apply during pregnancy.
The weight range for your height
The calculator also converts the 18.5 to under 25 range back into weights at your height. At 175 cm that is 56.7 kg up to but not including 76.6 kg. This is simply the same formula run backwards. It is not a recommended or ideal weight, and a weight inside it does not guarantee good health, just as one outside it does not guarantee poor health. If you want a body-composition view, try the body fat calculator or the lean body mass calculator, and browse the full fitness and health calculators.
BMI Prime and the Ponderal Index
Two related numbers sometimes appear next to BMI, and this calculator shows both. BMI Prime is your BMI divided by 25, the top of the CDC healthy weight range. A BMI Prime below 1.0 means a BMI below 25, and a value above 1.0 means above it. For 70 kg at 175 cm the BMI Prime is 0.91. It is the same information as BMI on a different scale, which some people find easier to read as "how far from the upper limit".
The Ponderal Index is weight divided by height cubed, in kilograms per cubic meter. It was proposed because BMI can drift at very short or very tall heights, where weight does not scale exactly with the square of height. For 70 kg at 175 cm it is 13.1 kg/m³. The CDC adult categories are defined on BMI, not on the Ponderal Index, so this number is for reference and comparison only and has no official category thresholds on this page.
BMI, body fat and waist size
BMI cannot see what the weight is made of. Two people of the same height and weight have the same BMI even if one has much more muscle and the other much more fat. That is why a body-fat estimate, a waist measurement or a clinician's assessment adds useful information. The body fat calculator compares two common estimation methods, and the waist-to-height calculator shows how a single waist and height measurement can be turned into a ratio. Neither replaces a medical assessment, but together with BMI they give a more rounded picture than any single number.
Getting a reliable measurement
BMI is only as good as the two numbers behind it. Weigh yourself at a consistent time of day, ideally in light clothing, and use the same scale. Measure height without shoes, standing straight against a wall. People often overstate their height and understate their weight when they estimate, and a difference of a couple of centimeters or kilograms can move a BMI by a few tenths. If your result sits close to a category boundary, treat the category as approximate rather than a hard line.
Using BMI sensibly over time
A single BMI reading is a snapshot. Trends over months and years tell you more than one value, particularly for adults whose weight is changing without a clear reason. If you notice an unexplained change, speak to a healthcare professional rather than relying on a calculator. If you want to see what a weight range looks like at your height, use the range shown in the results table, and for energy needs see the calorie and TDEE calculator.
Frequently asked questions
What is a healthy BMI for adults?
The CDC labels a BMI from 18.5 up to but not including 25 as the healthy weight range for adults aged 20 and older. It is a population screening range, not a personal target.
Is BMI accurate for athletes?
It can be misleading. Muscle weighs more than fat per unit volume, so a muscular athlete can land in the overweight or obesity category with low body fat. A body-fat estimate or a clinician assessment gives a better picture.
Does the BMI formula differ for men and women?
No. The adult BMI formula and the CDC categories are the same for men and women, which is why this calculator does not ask for sex.
Can I use this calculator for my child?
No. Children and teens aged 2 to 19 are assessed with BMI-for-age percentiles. Ask a pediatrician or use a growth chart designed for their age and sex.
Why does my BMI show 25.0 but the category says healthy weight?
The displayed BMI is rounded to one decimal place, but the category is chosen from the unrounded value. A true BMI of 24.96 shows as 25.0 and is still below the overweight cut-off.
Does a high BMI mean I have a health problem?
Not by itself. BMI is a screening step. Blood pressure, blood sugar, cholesterol, waist size, fitness and family history all matter, and a healthcare professional can interpret them together.
Sources and limits of this page
Categories and the screening wording follow the CDC page About Adult BMI (checked 9 November 2026). This page is educational information, not medical advice, and it does not diagnose, treat or recommend a weight loss or gain plan. Talk to a qualified healthcare professional about your own health.